Such cooling apparatus is widely used in industry, for example in the freezing of food. A liquefied gas, typically liquid nitrogen, may for example be used to cool food in a tumbler or screw conveyor, or may be used to freeze food in a tunnel or a so-called immersion freezer. In the example of the freezing tunnel, food is advanced on a conveyor through a chamber in the form of a tunnel into which liquid nitrogen is injected. Typically, the liquid nitrogen is directed at the food and extracts heat therefrom as it vaporizes. A fan or fans are employed to provide a flow of cold nitrogen vapor through the tunnel in a direction opposite to that in which the food is advanced through the tunnel and this flow of cold nitrogen vapor is also able to extract heat from the food. In order to prevent or limit the amount of cold nitrogen vapor from spilling out of the ends of the tunnel, a fan is employed to extract the cold nitrogen vapor from the tunnel through an outlet usually positioned in its roof near its entrance. Since liquid nitrogen vaporizes at a temperature of minus 196.degree. C., the temperature of the vapor extracted from the tunnel is typically well below freezing point of water even though there has been heat exchange between the vapor and the food (or other articles or material being advanced through the tunnel). It is therefore a common practice to dilute the extracted vapor with air upstream of the extractor fan so as to increase the temperature of the vapor to above freezing point and thereby avoid deposition of ice on the fan itself. Some dilution takes place in the tunnel since the fan induces a flow of air into the tunnel. However, this dilution is not sufficient to give an adequate increase in temperature. Accordingly, the ducting by which the fan is placed in communication with the outlet from the tunnel is provided with an adjustable inlet for ambient air. Typically, this inlet is designed so as to enable the fan to draw in a flow of ambient air into the ducting at a rate 3 or 4 times that at which the mixture of cold nitrogen vapor and air enters the ducting from the outlet of the freezing tunnel.
There are a number of disadvantages associated with such exhaust gas extractions systems. In particular, the extraction duct needs to be of greater diameter than it would otherwise have to be in order to cope with the induced air flow. Moreover, the refrigerative capacity of the extracted nitrogen vapor is wasted. In addition, if the ambient air has been conditioned, (a common practice in food processing factories), extracting air with the nitrogen vapor effectively reduces the overall efficiency of the air conditioning system. A further disadvantage is that practical problems arise with the control of the extraction system: the operation of the fan is typically linked to a valve controlling the flow of liquid nitrogen into the tunnel. Since the tunnel may be operated with a widely varying range of belt loadings the temperature of the nitrogen vapor at the outlet can vary widely even though the valve is controlled so as to give a desired product temperature at the tunnel exit. Accordingly, in practice, difficulties arise in continuously maintaining the fan free of ice.